The present invention generally relates to a method and apparatus used in metal melting, refining and processing, for example, steel making in an electric arc furnace (EAF), and more particularly, to a method and apparatus for the introduction of chemical energy and particulates, for example, carbon particles entrained in a carrier gas, in an EAF.
Electric arc furnaces (EAFs) make steel by using an electric arc to melt one or more charges of scrap metal which is placed within the furnace. Modern EAFs may also make steel by melting DRI (direct reduced iron) combined with the hot metal from a blast furnace. In addition to the electrical energy of the arc, chemical energy is provided by auxiliary burners using fuel and an oxidizing gas to produce combustion products with a high heat content to assist the arc.
If the EAF is used a scrap melter, the scrap burden is charged by dumping it into the furnace through the roof opening from buckets which also may include charged carbon and slag forming materials. A similar charging method using a ladle for the hot metal from a blast furnace may be used along with injection of the DRI by a lance to produce the burden.
In the melting phase, the electric arc and burners melt the burden into a molten pool of metal, termed an iron carbon melt, which accumulates at the bottom or hearth of the furnace. Typically, after a flat bath has been formed by melting of all the burden introduced, the electric arc furnace enters a refining and/or decarburization phase. In this phase, the metal continues to be heated by the arc until the slag forming materials combine with impurities in the iron carbon melt and rise to the surface as slag. During the heating of iron carbon melt it reaches the temperature and conditions when carbon in the melt combines with oxygen present in the bath to form carbon monoxide bubbles which is commonly termed as “carbon boil.” Generally, flows of oxygen are blown into the bath with either lances or burner/lances to produce a decarburization of the bath by the oxidation of the carbon contained in the bath.
The resulting decarburization reduces the carbon content of the bath to a selected level. If an iron carbon melt is under 2% carbon it becomes steel. Except for operations using the hot metal from the Blast furnaces, the EAF steel making processes typically begin with burdens having less than 1% carbon. The carbon in the steel bath is continually reduced until it reaches the content desired for producing a specific grade of steel, down to less than 0.1% for low carbon steels.
With the imperative to decrease steel production times in electric arc furnaces, it becomes necessary to deliver effective decarburizing oxygen to the iron carbon melt as early in the steel making process as possible. Conventional burners mounted on the water cooled side walls of the furnace generally must wait until the melting phase of the process is substantially complete before starting high velocity injection of oxygen for the decarburization process. These burners can not deliver effective high velocity oxygen to the bath early in to the melting cycle because unmelted scrap is in the way of the injection path and would deflect the oxygen flow. The bottom of the electric arc furnace is spherical shaped and the melted scrap forms the melt in the middle of the furnace first and then it rises filling up the sides.
Therefore, it would be highly advantageous to reduce the melting phase of an electric arc furnace so that high velocity oxygen and carbon could be injected sooner and decarburize the melt faster.
One way to shorten the melting phase is to add substantially more energy with the burners at early times in the melting phase to melt the scrap faster. There are, however, practical considerations with conventional side wall mounted burners that limit the amount of energy which can be introduced into the furnace and the rate at which it can be used efficiently. The location of a conventional burners is subject to flashback. When scrap is initially loaded into the furnace, because it is located very near the flame face and oxygen jet of the burner, the danger of a flash back of the flame against the side wall where the burner is mounted is significant. The panels where the burners are mounted are typically water cooled and a burn through of a water carrying element in an electric arc furnace is a safety concern, as well as a production loss. To alleviate this concern, many fixed burners are run at less than rated capacity until the scrap is melted some distance away from the face of the burner. Only after the burner face has been cleared does the burner operate to deliver its maximum energy.
Another problem to increasing the energy added during the early part of the melting phase is that the flame of the burner is initially directed to a small localized area of the scrap on the outside of the scrap burden. It is difficult to transfer large amounts of energy from the burner by this localized impingement to the rest of the scrap efficiently. Until the burner has melted the scrap away from its face and has opened a larger heat transfer area, increasing a burner to maximum output would result in overheating and melting scrap pieces together producing the problems for the next stage of the EAF operation.
Therefore, it would be advantageous to be able to increase the amount of energy applied by a burner during the early part of the melting phase which did not produce a risk of flash back for the water cooled panels of the upper shell of the furnace. It would also be advantageous to use this increased amount of energy more efficiently and to transfer increased portions of the energy to the scrap burden without scrap agglomeration.
Conventionally, oxygen is blown or injected into the iron carbon melt where it reacts with the carbon in the molten bath to lower the carbon content to the level desired for the end product. In general, the rate of decarburization in an electric arc furnace is determined by the carbon concentration of the iron carbon melt, the oxygen injection rate and the surface area of the reactions sites. At higher bath carbon concentrations, the reaction rate is not significantly limited by the availability of carbon to enter the reaction. However, as the bath carbon decreases to concentrations under approximately 0.15%-0.20% of carbon, it becomes increasingly difficult to achieve an acceptable rate. This is because the carbon concentration of the bath becomes the decarburization rate determining factor. The decarburization rate, after the critical carbon content has been reached, is dominated by mass transfer of the carbon and the carbon concentration.
The prior art practice to decarburize an iron carbon melt is characterized by the localized application of a large volume of oxygen by means of devices such as lances and burner/lances. Due to the localized nature of this process, the decarburization rate depends on the rate of oxygen injection to the bath, the carbon concentration and the mass transfer of carbon to the reaction area. At lower carbon level contents, the iron oxide concentration in the slag near to the oxygen introduction area reaches levels greater than equilibrium would allow, due to depleted local carbon concentration and poor mass transport. This causes greater refractory erosion, loss of iron yield, increased requirements for alloys, and a low efficiency of oxygen utilization.
Therefore, it would be advantageous to provide a method and apparatus to supply oxygen for efficient decarburization of the iron carbon melt at all carbon concentrations. A method that increased the number of reaction zones and supplied significantly more effective oxygen early in the process would be advantageous because it would shorten the duration of decarburization. Particularly important is the efficiency of the oxygen supply after the iron carbon melt reaches a low carbon content in order to maximize the decarburization rate, without over oxidizing the slag and producing excess amounts of FeO. This would reduce operating costs by improving oxygen efficiency, reducing excess iron oxidation, improving alloy recovery, and increasing productivity.
The conventional oxygen injection equipment that has been used for decarburization is not generally suited for efficient introduction of oxygen into an iron carbon melt. The use of retractable consumable or water cooled lances through the slag door opening, or through the side wall, is always limited by the space available to position the equipment around the furnace. Its location is usually only practical in the quadrant of the furnace shell near the slag door. The basic furnace design, required manipulator movement, the size of the manipulator and the necessity of operators to observe the manipulator operation dictate the location of the manipulator. The design is also responsible for the introduction of a substantial amount of cold ambient air into the process through the slag door or side wall opening during manipulation of the moveable lance. These large amounts of cold air reduce the efficiency of the process and also contribute to a nitrous oxide increase in the furnace atmosphere. There is also a significant delay in moving the lance into the furnace through the scrap burden. The scrap must be melted in front of the lance before it can advanced into the hot reaction zone of the furnace where it can deliver effective oxygen.
Fixed oxygen injection equipment such as a burner/lance mounted on the side wall water cooled panels, or upper shells of the furnace are positioned a significant distance away from the iron carbon melt. That distance is generally determined by the geometry of the furnace side wall with respect to the transition from the upper shell to the lower shell of the furnace which forms a step. The water cooled part of the upper shell where the burner/lances have been located is mounted on the lower shell or refractory, but typically about 15-24 inches back from the hot face of the refractory. Because a fixed burner/lance has had to fire over this step, the traditional fixed wall oxygen injection equipment had to be located about 45 inches above the molten bath in an attempt to deliver oxygen with the optimum angle of impingement. This distance and the angle requires the length of the injected stream of oxygen to be about 65 inches or longer.
It is very difficult to deliver 1000 of an oxygen stream effectively to a reaction zone at these distances. The amount of effective delivery of a high velocity (high kinetic energy) oxygen stream to the iron carbon melt is proportional to the area of the oxygen injector opening (in the case of a converging-diverging nozzle the area of the nozzle's throat) and the distance the oxygen jet travels to the iron carbon melt. Thus, increasing the area of the nozzle throat increases the total amount of effective oxygen reaching the iron carbon melt, but may also result in an increase of unused oxygen in the furnace atmosphere. Another method of enhancing the effectiveness of an oxygen stream for decarburization has been to shroud it with the products of combustion, or other gases. The shrouding tends to maintain the stream together over a longer distance thereby increasing its penetrating power. In spite of the effectiveness gained by shrouding, it still has the limitation of how far the gases can travel without significant energy loss. Locating the oxygen injection device far from the melt results in a significant amount of the oxygen being lost to the furnace environment and causing several detrimental effects on operations. Initially, there is the increased cost of the shrouding gases and specialized equipment to form the shroud. The excess oxygen causes damage to the side wall panels, erosion of the shell refractory, development of excessive iron oxide in the slag, excessive electrode oxidation, reduction in the delta life, and may cause over heating of the furnace evacuation system.
Moreover, conventional oxygen injection equipment that has been used for decarburization is not generally suited to varying the oxygen supply rate over substantial ranges. Fixed oxygen injection equipment such as burner/lances mounted on the side wall panels of the furnace have the problem that they are positioned some distance away from the surface of the iron carbon melt. These fixed lances obtain their oxygen injection capability by a supersonic or high velocity nozzle which accelerates the oxygen such that its kinetic energy is enough to penetrate the surface of the iron carbon melt even from considerable distances. If the flow rates of these injectors are reduced significantly, the high velocity nozzles will not impart enough gas velocity to the oxygen to penetrate and create an efficient reaction zone for decarburization.
The introduction of particulates in EAFs has also increased with the requirements for the efficient processing of iron carbon melts and are usually introduced for slag production. The production of a correct slag composition for the iron carbon melt during the refining phase is important in achieving desired steel chemistry and in cleaning the steel of impurities. Foamy slag practice where the slag entrains gas bubbles, usually CO gas bubbles, and expands in volume to cover the electrodes) of the furnace and protect furnace components from the arc radiation is very desirable. Particulates, such as CaO and MgO, have been introduced to form slag and correct its chemistry to provide a good basis for slag foaming. Slag foaming is generally accomplished by the introduction of particulate carbon into the bath where it reduces FeO to Fe in an endothermic reaction producing CO bubbles which expand the volume of the slag and cause it to foam. The foamed slag acts as a blanket to hold in heat for the process and to shield furnace components from the radiation of the electric arc.
Also particulate carbon has been introduced into the EAF environment for the chemical adjustment of the carbon content an iron carbon melt. Normally, carbon is added to a melt for cleaning purposes or to increase the carbon content if the carbon content of the original iron burden melted had been too low for the grade of steel desired.
Carbon has also been added to a slag which has high percentage of FeO to recover Fe from the slag to increase the yield of the steel. U.S. Pat. No. 4,362,556 issued to Kashida describes the process of recovering Fe from the slag by reducing it with introduced particulate carbon. The carbon introduction is disclosed as being lanced with a pipe, either by itself or simultaneously with the introduction of oxygen.
Carbon has in the past been introduced into the EAF by a number of methods including adding it to the buckets of scrap which are being melted or by shoveling it through openings in the EAF, including ones in the roof, sidewalls and the slag door. This has proved inefficient and other methods, such as moveable lances and fixed multimode burners, are now used. U.S. Pat. No. 5,599,375 issued to Gitman, et al. illustrates a multimode burner which injects a simultaneous mixture of oxygen and particulate carbon in an EAF. U.S. Pat. No. 4,986,847 issued to Knapp discloses a slag door manipulator which simultaneously intersects streams of oxygen and carbon before injection into the furnace.
The incorporated Shver applications disclose a furnace apparatus mounting configuration which allows a multimode burner/lance to be moved closer to the step of the furnace. The mounting enclosure and burner/lance configuration moves the burner flame away from the sidewall panel to eliminate the chance for flashback and water cooled panel damage. A more aggressive oxygen lancing practice can be used without risk of damaging the sidewall panel and a more optimal oxygen consumption can be achieved.
Similarly to the conventional burner/lance mounting configuration, a carbon injection stream is required to travel the same large distance that a conventional oxygen jet must travel. Often the suction of the direct evacuation system is strong enough to disrupt the carbon stream as it travels from the sidewall to the melt and thereby reduces the effectiveness of slag foaming. The large distance which the conventional carbon injection stream must travel also reduces the velocity and energy with which it may penetrate the slag. Slag foaming is much more effective when the carbon stream can produce an intense agitation of the slag at its place of introduction.
Therefore, it would be advantageous to provide a particulate injection process for EAF steelmaking, especially particulate carbon, which will introduce the particulates low in the furnace and close to the slag/metal interface. The introduction of particulates in this manner will maximize the kinetic energy of the stream for penetrating and agitating the slag. It would also be advantageous for the particulate injection to be accompanied by the injection of burner flames and decarburizing oxygen low in the furnace and close to the slag metal interface.
Prior art lances or burner/lances which simultaneously inject oxygen and carbon have the problem of being located far from the melt and other drawbacks. The oxygen and carbon are generally mixed at the end of two nozzles by intersecting the flows far away from the iron carbon melt. Because the intersection angle is fixed at the time of mounting, these conventional carbon and oxygen injection apparatus are only aligned or aimed to be effective when the iron carbon melt is at one level. This level is usually chosen as the designed flat bath or fully melted level of the furnace. However, no furnace actually operates at that level as most are either overloaded or under-loaded to some degree in day to day operation. The designed level even changes as the refractory of the furnace wears being higher at the start of a refractory regime and lower at the end. Further, before a full scrap burden level is reached in normal EAF operation, several scrap buckets must go through the melting cycle and it is not until the last bucket is entirely melted that the full level of the furnace is even approached. Other conventional steel making processes have variable level baths, such as melting direct reduced iron (DRI), or a ConSteel process. Therefore, these simultaneous carbon and oxygen injection systems are not very efficient over much of the scrap melting steel making process and almost ineffective for many other conventional steel making processes.
Therefore, it would be of advantage to provide a particulate injection process for EAF steel making where the flows of oxygen and carbon were substantially parallel to each other and did not intersect. A substantially parallel introduction of each stream low in the furnace and close to the slag/metal interface would provide effective decarburization and effective slag foaming to begin early in the process and at low bath levels. Further, the introduction of flows which did not intersect also would be effective over a wide range of bath levels.
Another drawback of the simultaneous oxygen and carbon systems which intersect the flows is the use of the carbon as a fuel and not for metallurgical purposes. This drawback increases the farther away they are introduced from the melt and their misalignment. The most efficient use of supersonic oxygen is to decarburize the melt and the most efficient use of carbon injection is to foam the slag and reduce the FeO in the slag. When mixed externally from the melt and slag, the oxygen and carbon combine to produce a flame leaving less of these elements for their intended uses. Further, their combined presence in one reaction zone slows the principle reactions desired (the decarburization of Fe and the reduction of FeO) as they are more reactive with each other than with their intended metallurgical combinations. The exothermic reaction of the carbon and oxygen merely produces an insignificant amount of uncontrollable heat while reducing the efficiency of and slowing the more desired processes.
Therefore, it would be advantageous to provide a particulate injection process for an EAF in which the reaction zone for decarburization is in close proximity to, but separate from the reaction zone for the reduction of FeO. This will maximize the primary reaction for the carbon, while at the same time ensuring efficient FeO reduction and maximum slag foaming. It would also be advantageous, however, if the two zones worked effectively together where the FeO produced by the decarburization zone was actively reduced in the particulate reaction zone.